1. Field of the Invention
The present invention relates to an imaging apparatus for a microscope which takes an observed image on the microscope.
2. Description of the Related Art
Conventionally, an imaging apparatus for a microscope is used for taking an observed image on the microscope with the use of an image taking means, such as a CCD, as a peripheral device.
Incidentally, since different brightness occurs from spot to spot on a sample depending upon the kinds of samples or the observation methods, the observed image sometimes involves dotted high and low bright spots.
When light is measured, on the average, over a whole image-taking range of such an observed image and an image is taken with an exposure time based on the measured value, an image with an incorrect exposure is obtained because a greater adverse effect is gained from a background than the dotted radiation spots. For example, an observed image of a fluorescent sample is dotted with high brightness radiation spots in a dark background. If, therefore, light is measured, on the average, over a whole image-taking range, the exposure time is simply calculated from the measured value with the use of a known calculation equation and an image is taken based on the exposure time, then the dark background is brightly imaged and a high fluorescent brightness spot as an object of interest is over-exposed.
Conventionally, a method disclosed in JPN PAT APPLN KOKAI Publication No. 5-56331 is provided as the image-taking technique using such a subject as an object of image taking. This method disclosed in the KOKAI Publication is used to image a subject with a spot light formed at a portion of a dark background. First, the light measuring range is divided into 24 areas, the brightness levels of the respective areas are detected and a brightness histogram is prepared from the detected brightness levels. And from the brightness histogram the upper N (=2) areas are extracted and AE control is performed based on the N areas only. The brightness levels of the N areas are averaged and the average brightness level is compared with an initially set threshold value y1. If the average brightness level is higher than y1, then with a gain fixed 0 dB an iris is so controlled as to set the average brightness level to 50 IRE and, by doing so, AE control is performed. If, on the other hand, the average brightness level is lower than y1, then with the iris fully opened a gain is so controlled as to set the average brightness level to 50 IRE and, by doing so, AE control is performed.
Incidentally, the method disclosed in the JPN PAT APPLN KOKAI Publication No. 5-56331 adjusts the brightness level by the gain control at the time of taking an image corresponding to high brightness spots dotted in a dark background such as a fluorescent image taking. In this case, the signal-to-noise ratio is adversely affected. Since the areas used under the AE control are fixed to the upper N areas, defects or dirt on the sample are higher in brightness than radiance spots as an object of image taking and the radiance spots become under-exposed.